1. Field of the Invention
The invention relates to an apparatus for transferring a monitor signal, to be used in a system for transferring an optical signal.
2. Description of the Related Art
FIG. 1 is a block diagram of a conventional apparatus for transferring a monitor signal, to be used in an optical-signal transferring system.
The apparatus is comprised of an optical branching filter 1, an optical band-pass filter 2, a light receiver 3, a monitor control signal processor 4, a laser driver 5, a laser module 6, an optical coupler 7, and an optical amplifier 8 optically connected in series between the optical branching filter 1 and the optical coupler 7.
The optical band-pass filter 2, the light receiver 3, the monitor control signal processor 4, the laser driver 5, and the laser module 6, are optically connected in series between the optical branching filter 1 and the optical coupler 7, and in parallel with the optical amplifier 8. The monitor control signal processor 4 receives a monitor control signal through a monitor-signal input terminal 11.
The optical branching filter 1 is optically connected to a signal input terminal 9, and the optical coupler 7 is optically connected to a signal output terminal 10.
The apparatus having the above-mentioned structure operates as follows.
An optical multiple signal in which a primary signal having a wavelength of λ s and a secondary signal monitoring a wavelength and having a wavelength of λ sv are multiplexed each other is input into the optical branching filter 1 through the signal input terminal 9. Since the wavelength of λ sv of the secondary signal is out of a gain band width of the optical amplifier 8, the secondary signal is directed to the optical band-pass filter 2 by the optical branching filer 1 upstream of the optical amplifier 8.
The secondary signal is input into the light receiver 3 through the optical band-pass filter 2. The light receiver 3 converts the received secondary signal into an electric signal. The thus produced electric signal is transferred to the monitor control signal processor 4, and processed by the monitor control signal processor 4 together with a monitor control signal input into the monitor control signal processor 4 through the monitor control signal input terminal 11.
The monitor control signal having been processed by the monitor control signal processor 4 is output to the laser driver 5, and then, is modulated in the laser module 6.
The thus modulated monitor control signal is combined at the optical coupler 7 with the primary signal having been amplified in the optical amplifier 8. The thus combined monitor control signal and primary signal is output through the signal output terminal 10.
The above-mentioned apparatus is accompanied with a problem that the apparatus has to include the laser module 6 by the number equal to the number of the optical amplifiers 8. Since the laser module 6 is quite expensive, the apparatus costs too much in fabrication thereof.
For instance, Japanese Unexamined Patent Publications Nos. 3-214936 and 9-284234 have suggested an apparatus for transferring a monitor signal, to be used for an optical repeater. However, the suggested apparatuses include a modulator for modulating an input signal. This means that the suggested apparatuses have the same problem as the problem of the conventional apparatus illustrated in FIG. 1.